Set algorithm for phase change memory cell

ABSTRACT

Memory devices and methods for operating such devices are described herein. A method is described herein for operating a memory cell comprising phase change material and programmable to a plurality of resistance states including a high resistance state and a lower resistance state. The method comprises applying a first bias arrangement to the memory cell to establish the lower resistance state, the first bias arrangement comprising a first voltage pulse. The method further comprises determining whether the memory cell is in the lower resistance state, and if the memory cell is not in the lower resistance state then applying a second bias arrangement to the memory cell. The second bias arrangement comprises a second voltage pulse having a pulse height greater than that of the first voltage pulse.

RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/345,384 filed on 29 Dec. 2008 which application isincorporated herein by reference.

PARTIES TO A JOINT RESEARCH AGREEMENT

International Business Machines Corporation, a New York corporation,Macronix International Corporation, Ltd., a Taiwan corporation, andInfineon Technologies A.G., a German corporation, are parties to a JointResearch Agreement.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to memory devices based on phase changebased memory materials, including chalcogenide based materials and onother programmable resistive materials, and methods for operating suchdevices.

2. Description of Related Art

Phase change based memory materials, like chalcogenide based materialsand similar materials, can be caused to change phase between anamorphous state and a crystalline state by application of electricalcurrent at levels suitable for implementation in integrated circuits.The generally amorphous state is characterized by higher electricalresistivity than the generally crystalline state, which can be readilysensed to indicate data. These properties have generated interest inusing programmable resistive material to form nonvolatile memorycircuits, which can be read and written with random access.

In phase change memory, data is stored by causing transitions in anactive region of the phase change material between amorphous andcrystalline states. FIG. 1 is a graph of memory cells comprising phasechange material and programmable to a plurality of resistance statesincluding a high resistance reset (erased) state 102 and at least onelower resistance programmed (set) state 100, each state havingnon-overlapping resistance ranges.

The difference between the highest resistance R₁ of the lower resistancestate 100 and the lowest resistance R₂ of the high resistance resetstate 102 defines a read margin 101 used to distinguish cells in thelower resistance state 100 from those in the high resistance state 102.The data stored in a memory cell can be determined by determiningwhether the memory cell has a resistance corresponding to the lowerresistance state 100 or to the high resistance state 102, for example bymeasuring whether the resistance of the memory cell is above or below athreshold resistance value R_(SA) 103 within the read margin 101.

The change from the high resistance state 102 to the lower resistancestate 100, referred to as set (or program) herein, is generally a lowercurrent operation in which current heats the phase change material abovea transition temperature to cause transition from the amorphous to thecrystalline state. The change from lower resistance state 100 to thehigh resistance state 102, referred to as reset herein, is generally ahigher current operation, which includes a short high current densitypulse to melt or breakdown the crystalline structure, after which thephase change material cools quickly, quenching the phase change processand allowing at least a portion of the phase change material tostabilize in the amorphous state.

In order to reliably distinguish between the high resistance state 102and the lower resistance state 100, it is important to maintain arelatively large read margin 101. However, since the active regionundergoes a phase change as a result of heating, during operation issuessuch as compositional changes in the phase change material within theactive region can result in the formation of a high resistance interfacewithin the electrical conduction path of the memory cell. This highresistance interface can result in a “set failure mode” in which thelower current set operation cannot successfully reduce the resistance ofthe memory cell below the threshold resistance value R_(SA), resultingin reliability issues and bit errors for those memory cells.

It is therefore desirable to provide a memory device and methods foroperating such devices addressing the set failure mode and resulting inimproved reliability and improved data storage performance.

SUMMARY OF THE INVENTION

A method is described herein for operating a memory cell comprisingphase change material and programmable to a plurality of resistancestates including a high resistance state and at least one lowerresistance state. The method comprises applying a first bias arrangementto the memory cell to establish the lower resistance state, the firstbias arrangement comprising a first voltage pulse. The method furthercomprises determining whether the memory cell is in the lower resistancestate, and if the memory cell is not in the lower resistance state thenapplying a second bias arrangement to the memory cell to establish thelower resistance state. The second bias arrangement comprises a secondvoltage pulse having a pulse height greater than that of the firstvoltage pulse.

A memory device as described herein comprises a memory cell comprisingphase change material and programmable to a plurality of resistancestates including a high resistance state and a lower resistance state.The memory device further comprises bias circuitry adapted to apply biasarrangements to the memory cell. The bias arrangements include a firstbias arrangement to establish the lower resistance state, the first biasarrangement comprising a first voltage pulse. The bias arrangements alsoinclude a read bias arrangement to determine whether the memory cell isin the lower resistance state after the set bias arrangement. The biasarrangements further include a second bias arrangement to establish thelower resistance state, the second bias arrangement comprising a secondvoltage pulse having a pulse height greater than that of the firstvoltage pulse.

Memory devices and methods for operating such devices described hereinaddress the set failure mode and result in improved endurance,reliability and data storage performance. Set operations describedherein comprise applying lower voltages across the phase change materialof memory cells to establish the lower resistance state and onlyapplying higher voltages across the phase change material when the lowervoltage is insufficient to set the memory cell.

Other aspects and advantages of the present invention can be seen onreview of the drawings, the detailed description, and the claims whichfollow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of memory cells comprising phase change material andprogrammable to a plurality of resistance states including a highresistance reset state and at least one lower resistance programmedstate.

FIGS. 2A-2C illustrate schematic diagrams of three prior art phasechange memory cells having a phase change memory element coupled to aselect device.

FIGS. 3A-3E illustrate cross-sectional views of prior art configurationsof memory elements.

FIG. 4 is a simplified block diagram of an integrated circuit in whichthe set operations described herein can be implemented.

FIG. 5 illustrates a portion of the memory array.

FIG. 6 is a flow diagram of a first embodiment of a set operation forprogramming a memory cell from a higher resistance state to a lowerresistance state.

FIG. 7 illustrates an embodiment of a timing diagram of the setoperation of FIG. 6.

FIG. 8 illustrates heuristic curves of the temperature versus time forportions of the set operation of FIG. 6.

FIGS. 9A-9C illustrate some alternative pulses for the set biasarrangement of the set operation of FIG. 6.

FIG. 10 is a flow diagram of a second embodiment of a set operation forprogramming a memory cell from a higher resistance state to a lowerresistance state

FIG. 11 illustrates an embodiment of a timing diagram of the setoperation of FIG. 10.

FIG. 12 illustrates heuristic curves of the temperature versus time forportions of the set operation of FIG. 10.

FIG. 13 is a flow diagram of an embodiment of a set operation forprogramming a group of memory cells from a higher resistance state to alower resistance state.

DETAILED DESCRIPTION

The following description of the disclosure will typically be withreference to specific structural embodiments and methods. It is to beunderstood that there is no intention to limit the disclosure to thespecifically disclosed embodiments and methods, but that the disclosuremay be practiced using other features, elements, methods andembodiments. Preferred embodiments are described to illustrate thepresent disclosure, not to limit its scope, which is defined by theclaims. Those of ordinary skill in the art will recognize a variety ofequivalent variations on the description that follows. Like elements invarious embodiments are commonly referred to with like referencenumerals.

FIGS. 2A-2C illustrate schematic diagrams of three prior art phasechange memory cells having a phase change material memory element 220(represented in the Figures by a variable resistor) and coupled to anaccess device such as a transistor or diode.

FIG. 2A illustrates a schematic diagram of a prior art memory cell 200including a field effect transistor (FET) 210 as an access device. Aword line 240 extending in a first direction is coupled to the gate ofthe FET 210 and a memory element 220 couples the drain of the FET 210 toa bit line 230 extending in a second direction.

FIG. 2B illustrates a schematic diagram of memory cell 202 similar tothat of FIG. 2A except that the access device is implemented as abipolar junction transistor (BJT) 212, while FIG. 2C illustrates aschematic diagram of a memory cell 204 similar to that of FIG. 2A exceptthat the access device is implemented as a diode 214.

Reading or writing can be achieved by applying suitable voltages to theword line 240 and bit line 230 to induce a current through the memoryelement 220. The level and duration of the voltages applied is dependentupon the operation performed, e.g. a reading operation or a writingoperation.

FIGS. 3A-3E illustrate cross-sectional views of prior art configurationsfor memory element 220.

FIG. 3A is a simplified cross-sectional view illustrating a firstconfiguration for memory element 220 coupled to first and secondelectrodes 312, 314. The first electrode 312 may, for example, becoupled to a terminal of an access device such as a diode or transistor,while the second electrode 314 may be coupled to a bit line.

A dielectric spacer 313 having a width 315 separates the first andsecond electrodes 312, 314. The phase change material of memory element220 extends across the dielectric spacer 313 and contacts the first andsecond electrodes 312, 314, thereby defining an inter-electrode pathbetween the first and second electrodes 312, 314 having a path lengthdefined by the width 315 of the dielectric spacer 313. In operation, ascurrent passes between the first and second electrodes 312, 314 andthrough the memory element 220, the active region 318 of the phasechange material of the memory element 220 heats up more quickly than theremainder of the memory element 220.

FIG. 3B is a simplified cross-sectional view illustrating a secondconfiguration for memory element 220 coupled to first and secondelectrodes 322, 324. The phase change material of the memory element 220has an active region 328 and contacts the first and second electrodes322, 324 at top and bottom surfaces 323, 329 respectively. The memoryelement 220 has a width 321 the same as that of the first and secondelectrodes 322, 324.

FIG. 3C is a simplified cross-sectional view illustrating a thirdconfiguration for memory element 220 coupled to first and secondelectrodes 332, 334, the phase change material of memory element 220having an active region 338. The first and second electrodes 332, 334are separated by dielectric spacer 335. The first and second electrodes332, 334 and the dielectric spacer 335 have a sidewall surface 331. Thephase change material of memory element 220 is on the sidewall surface331 and extends across the dielectric spacer 335 to contact the firstand second electrodes 332, 334.

FIG. 3D is a simplified cross-sectional view illustrating a fourthconfiguration for memory element 220 coupled to first and secondelectrodes 342, 344. The phase change material of memory element 220 hasan active region 348 and contacts the first and second electrodes 342,344 at top and bottom surfaces 343, 349 respectively. The memory element220 has a width 341 less than that of the first and second electrodes342, 344.

FIG. 3E is a simplified cross-sectional view illustrating a fifthconfiguration for memory element 220 coupled to first and secondelectrodes 354, 352. The first electrode 354 has a width 351 less thanwidth 353 of the second electrode 352 and memory element 220. Because ofthe difference between width 351 and width 353, in operation the currentdensity in the phase change material of memory element 220 is largest inthe region adjacent the first electrode 354, resulting in the activeregion 358 having a “mushroom” shape as shown in the Figure.

As was described above, in operation issues such as compositionalchanges in the phase change material within the active region can resultin formation of a high resistance interface within the conduction pathof the memory cell. The high resistance interface can result in a “setfailure mode” in which the resistance of the memory cell cannot bereduced to a resistance corresponding to a lower resistance state usinga lower voltage set operation, resulting in reliability issues and biterrors for those memory cells.

FIG. 4 is a simplified block diagram of an integrated circuit 400 inwhich the set operations (described in more detail below) can beimplemented, the set operations addressing the set failure mode andresulting in improved reliability and improved data storage performanceof the integrated circuit 400. The integrated circuit 400 includes amemory array 405 implemented using phase change memory cells (notshown). A word line decoder and drivers 410 having read, set, reset,reset verify, set verify, and high-voltage retry modes is coupled to andin electrical communication with a plurality of word lines 415 arrangedalong rows in the memory array 405. A bit line (column) decoder 420 isin electrical communication with a plurality of bit lines 425 arrangedalong columns in the array 405 for reading, setting, resetting, resetverify, set verify, and high-voltage retry of the phase change memorycells in array 405. Addresses are supplied on bus 460 to word linedecoder and drivers 410 and bit line decoder 420. Sense circuitry (Senseamplifiers) and data-in structures in block 430 are coupled to bit linedecoder 420 via data bus 435. Data is supplied via a data-in line 440from input/output ports on integrated circuit 400, or from other datasources internal or external to integrated circuit 400, to data-instructures in block 430. Other circuitry 465 may be included onintegrated circuit 400, such as a general purpose processor or specialpurpose application circuitry, or a combination of modules providingsystem-on-a-chip functionality supported by array 405. Data is suppliedvia a data-out line 445 from the sense amplifiers in block 430 toinput/output ports on integrated circuit 400, or to other datadestinations internal or external to integrated circuit 400.

The integrated circuit 400 includes a controller 450 for read, set, setverify, reset, reset verify, and high voltage retry modes. Thecontroller 450, implemented in this example using a bias arrangementstate machine, controls the application of bias arrangement supplyvoltages and current sources 455 for the application of biasarrangements including read, set, reset, reset verify, set verify, andhigh-voltage retry. The controller 450 is coupled to the senseamplifiers in block 430 via feedback bus 475, the controller 450controlling the bias arrangement supply voltages and current sources 455in response to output signals of the sense amplifiers. Controller 450may be implemented using special-purpose logic circuitry as known in theart. In alternative embodiments, controller 450 comprises ageneral-purpose processor, which may be implemented on the sameintegrated circuit to execute a computer program to control theoperations of the device. In yet other embodiments, a combination ofspecial-purpose logic circuitry and a general-purpose processor may beutilized from implementation of controller 450.

As shown in FIG. 5, each of the memory cells of array 405 includes anaccess transistor (or other access device such as a diode), four ofwhich are shown as memory cells 530, 532, 534, and 536 having respectivephase change memory elements 546, 548, 550, and 552 are illustrated inFIG. 5, representing a small section of an array that can includemillions of memory cells. The memory cells are programmable to aplurality of resistance states including a high resistance state and atleast one lower resistance state.

Sources of each of the access transistors of memory cells 530, 532, 534,536 are connected in common to source line 554 that terminates in asource line termination circuit 555, such as a ground terminal. Inanother embodiment the sources of the access devices are notelectrically connected, but independently controllable. The source linetermination circuit 555 may include bias circuits such as voltagesources and current sources, and decoding circuits for applying biasarrangements, other than ground, to the source line 554 in someembodiments.

A plurality of word lines 415 including word lines 556, 558 extend inparallel along a first direction. Word lines 556, 558 are in electricalcommunication with word line decoder 410. The gates of accesstransistors of memory cells 530, 534 are connected to word line 556, andthe gates of access transistors of memory cells 532, 536 are connectedto word line 558.

A plurality of bit lines 435 including bit lines 560, 562 extend inparallel in a second direction and are in electrical communication withbit line decoder 420. Memory elements 546, 548 couple the bit line 560to the respective drains of the access transistors of memory cells 530,532. Memory elements 550, 552 couple the bit line 562 to the respectivedrains of the access transistors of memory cells 534, 536.

It will be understood that the memory array 405 is not limited to thearray configuration illustrated in FIG. 5, and additional arrayconfigurations can also be used. Additionally, instead of MOStransistors, bipolar transistors or diodes may be used as access devicesin some embodiments.

In operation each of the memory cells 530, 532, 534, 536 store a datavalue depending upon the resistance of the corresponding memoryelements. The data value may be determined, for example, by comparisonof current on a bit line for a selected memory cell to that of asuitable reference current. In a memory cell having three or morestates, a plurality of reference currents can be established so thatdiffering ranges of bit line currents correspond to each of the three ormore states.

Reading or writing to a memory cell of array 405, therefore, is achievedby applying a suitable voltage to one of word lines 556, 558 andcoupling one of bit lines 560, 562 to a voltage so that current flowsthrough the selected memory cell including through the correspondingmemory element. For example, a current path 580 through a selectedmemory cell (in this example memory cell 532 and corresponding memoryelement 548 are selected) is established by applying voltages to the bitline 560, word line 558, and source line 554 sufficient to turn on theaccess transistor of memory cell 532 and induce current in path 580 toflow from the bit line 560 to the source line 554, or vice-versa. Thelevel and duration of the voltages applied is dependent upon theoperation performed, e.g. a reading operation or a writing operation.

In a reset (or erase) operation of memory cell 532, word line decoder410 facilitates providing word line 558 with a suitable voltage to turnon the access transistor of the memory cell 532. Bit line decoder 420facilitates supplying one or more voltage pulses to bit line 560 ofsuitable amplitude and duration to induce a current to flow thoughmemory element 548, thereby raising the temperature of at least theactive region above the transition temperature of the phase changematerial of the memory element 548 and also above the meltingtemperature to place at least the active region in a liquid state. Thecurrent is then terminated, for example by terminating the voltage pulseon the bit line 560 and the voltage on the word line 558, resulting in arelatively quick quenching time as the active region rapidly cools tostabilize to an amorphous phase. The reset operation can comprise one ormore pulses, for example comprising a pair of pulses.

In a read (or sense) operation of memory cell 532, word line decoder 410facilitates providing word line 558 with a suitable voltage to turn onthe access transistor of the memory cell 532. Bit line decoder 420facilitates supplying a voltage to bit line 560 of suitable amplitudeand duration to induce current to flow that does not result in thememory element 448 undergoing a change in resistive state. The currenton the bit line 560 and through the memory element 548 is dependent uponthe resistance of, and therefore the data state associated with, thememory element 548 of the memory cell 532. Thus, the data state of thememory cell may be determined, for example by comparison of the currenton bit line 560 with a suitable reference current by sense amplifiers ofsense circuitry 430.

FIG. 6 is a flow diagram of a first embodiment of a set operation 600for programming memory cell 532 from a higher resistance state to alower resistance state. FIG. 7 illustrates an embodiment of a timingdiagram of the set operation 600 of FIG. 6. As will be understood thetiming diagram of FIG. 7 is simplified and not necessarily to scale.

The set operation 600 for selected memory cell 532 begins at step 610.Step 610 may include, or in some embodiments be preceded by, a readoperation to determine if the selected memory cell 532 need to beprogrammed by the set operation 600. The read operation can beaccomplished by applying a read bias arrangement such as supplyingvoltages to word line 558 and bit line 560 sufficient to turn on theaccess transistor of the selected memory cell 532 and to induce currentto flow in path 580 on the bit line 560 and through the memory element548 to the source line 554 (which is terminated to ground in thisexample). The current is insufficient for the memory element 548 toundergo a change in resistive state, and the resistance of the memorycell 532 may be determined by comparison of the current on the bit line560 to a suitable reference current by sense amplifiers of block 430.

Referring to FIGS. 7 and 8, next at step 620 a first bias arrangement isapplied to the memory cell 532 to establish the lower resistance statein the memory cell 532. In the illustrated embodiment the first biasarrangement of step 620 comprises applying a voltage V_(WL-SET) to wordline 558 above the threshold voltage V_(th) of the access transistor ofthe selected memory cell 532, and applying a voltage pulse having apulse height of V_(SET) and pulse length 700 to the bit line 560 toinduce current to flow in path 580 and provide a first amount of energyto the phase change material of memory element 548.

As represented heuristically in curve 850 of FIG. 8, the first amount ofenergy provided to the phase change material of the memory element 548is sufficient to raise the temperature of at least a portion of theactive region of the memory element above the transition(crystallization) temperature 810 of the phase change material. Thefirst amount of energy causes at least a portion of the active region totransition into a crystalline phase, thereby establishing the lowerresistance state. As will be understood, the curve 850 is merelyillustrative and the actual shape of the curve 850 depends upon theproperties of the memory cell, the manner in which the set biasarrangement is applied to the memory cell, and the manner in which thephase change material heats up and cools down.

In the illustrated embodiment of FIGS. 6-8, the first bias arrangementof step 620 comprises a single pulse having a pulse height of V_(SET)and pulse width 700 applied to the bit line 560, although it will beunderstood that other set bias arrangements can alternatively be used.More generally, a set of one or more pulses may be applied to the bitline 560 and/or word line 558 and/or source line 554 to induce currentflow in path 580 to provide the first amount of energy to the phasechange material of memory element 548. The number of pulses and thepulse shapes, including the voltage levels and pulse widths, of thefirst bias arrangement can be determined empirically for eachembodiment. FIGS. 9A-9C illustrate some examples of the pulses of theset bias arrangement of step 620 that may be used in some alternativeembodiments.

Referring back to FIG. 6, the set method 600 continues to step 630. Atstep 630 the resistance of the selected memory cell is read to determinewhether the memory cell 232 has a resistance corresponding to the lowerresistance state. The read operation of step 630 applies a read biasarrangement to the memory cell 232. In the illustrated embodiment theread bias arrangement comprises maintaining the voltage V_(WL-READ) onthe word line 558 and applying a voltage pulse having a pulse height ofV_(READ) and pulse length 710 to the bit line 560 to induce current toflow in path 580, the current insufficient for the memory element 548 toundergo a change in resistive state. Other read bias arrangements mayalternatively be used.

For example, the resistance of the memory cell 532 may be determined bycomparison of the current on the bit line 560 to a suitable referencecurrent by sense amplifiers of block 430. Based on the comparison, anoutput signal of the sense amplifiers of block 430 indicating whetherthe memory cell has a resistance corresponding to the lower resistancestate is supplied to the controller 450 via feedback bus 475. Inresponse to the output signal, the controller 450 terminates the setoperation at step 650 if the selected memory cell 538 has a resistancecorresponding to the lower resistance state. Other techniques forterminating the set operation may alternatively be used.

If the resistance of the memory cell 532 is not in the lower resistancestate, the memory cell 532 has experienced a set failure. This failuremay come from the formation of a high resistance layer (or interface)within the electrical conduction path of the memory cell. A retry (orsecond) bias arrangement is then applied to the memory cell to createthe conduction path and establish the lower resistance state, the retrybias arrangement comprising a second voltage pulse across the phasechange material having a pulse height greater than the pulse heightV_(SET) of the set bias arrangement of step 620.

In the illustrated embodiment of FIG. 6, the retry bias arrangement ofthe set operation 600 begins at step 640 where a subsequent higher bitline voltage bias arrangement is applied to the memory cell.

Referring to FIGS. 7 and 8, in the illustrated embodiment the subsequentbias arrangement of step 640 comprises applying a voltage V_(WL-RETRY)to word line 558, and applying a voltage pulse having a pulse height ofV_(HIGH) and pulse length 720 to the bit line 560 to induce current toflow in path 580 and provide energy to the phase change material ofmemory element 548. As can be seen in FIG. 7, in the illustratedembodiment the pulse of step 640 has a pulse width less than that of thepulse of step 620 and has a pulse height greater than that of the pulseof step 620.

In embodiments the above mentioned V_(WL-SET), V_(WL-READ), V_(WL-RETRY)can be equal or different. For a typical set-up, using a higherV_(WL-READ) will increase the accuracy of the read operation, while alower V_(WL-SET) and V_(WL-RETRY) will prevent large current flowthrough the memory device when doing the set and retry operations.

As represented heuristically in curve 860 of FIG. 8, the pulse having apulse height V_(HIGH) across the phase change material of the memoryelement 548 is sufficient to breakthrough the high resistive layer andcreate a conduction path. As will be understood, the curve 860 is merelyillustrative and the actual shape of the curve 860 depends upon theproperties of the memory cell, the manner in which the subsequent biasarrangement is applied to the memory cell, and the manner in which thephase change material heats up and cools down.

In some embodiments the bias arrangement of step 640 is the same as thereset bias arrangement used for resetting the memory cell and issufficient to melt the active region and cause a transition to the highresistance state. Additionally, in some embodiments the subsequent biasarrangement may be current limited, for example by using a lowerV_(WL-RETRY), which may prevent damage of the memory device under thehigh bias condition once breakthrough of the high resistance layeroccurs. In FIG. 7 V_(WL-RETRY) is less than V_(WL-SET). As a result, insome embodiments the current induced through the phase change materialof the memory element during step 640 can be less than the currentinduced through the phase change material during step 620.

In the illustrated embodiment of FIGS. 6-8, the subsequent biasarrangement of step 640 comprises a single pulse having a pulse heightof V_(HIGH) and pulse width 720 applied to the bit line 560, although itwill be understood that other subsequent bias arrangements canalternatively be used. More generally, the subsequent bias arrangementof step 640 may comprise a set of one or more pulses applied to the bitline 560 and/or word line 558 and/or source line 554 to induce currentflow in path 580. The number of pulses and the pulse shapes, includingthe voltage levels and pulse width, of the subsequent bias arrangementcan be determined empirically for each embodiment.

In the illustrated embodiment of FIGS. 6-8, the word line voltages forthe different steps, including steps 630, 630, and 640 can havedifferent values.

Next, the retry (or second) bias arrangement of the set operation 600continues back to step 620 where the set bias arrangement is applied tothe memory 532.

The set operation 600 then continues to step 630 to determine whetherthe memory cell 232 has a resistance corresponding to the lowerresistance state. The set operation continues in the loop of iterativelyapplying the retry (or second) bias arrangement (steps 640, 620) anddetermining whether the memory cell 232 has a resistance correspondingto the lower resistance state (step 630) until the resistance of thememory cell 532 corresponds to the lower resistance state, or until apredetermined number of retries are made. In some alternativeembodiments the pulses of the retry bias arrangement may be changed foreach iteration. If in step 630 it is determined that the memory cell hasa resistance corresponding to the lower resistance state, the setoperation terminates at step 650.

In the illustrated embodiment of FIG. 6, if the memory cell 532 hasexperienced a set failure the retry bias arrangement comprises thesubsequent bias arrangement of step 640 combined with the first biasarrangement of step 620.

FIG. 10 illustrates a second embodiment of a set operation 1000 in whichthe retry (or second) bias arrangement 1040 does not include the firstbias arrangement of step 620, and instead includes the function ofsetting the memory cell 532. FIG. 11 illustrates an embodiment of atiming diagram of the set operation 1000 of FIG. 10. As will beunderstood the timing diagram of FIG. 11 is simplified and notnecessarily to scale.

Referring to FIGS. 11 and 12, in the illustrated embodiment the retry(or second) bias arrangement of step 1040 comprises applying a voltageV_(WL-RETRY) to word line 558, and applying a voltage pulse to the bitline 560 having a shape with initial pulse height of V_(HIGH) and atrailing edge in which the voltage on the bit line 560 decreases withtime as shown. The voltage pulse is sufficient to breakthrough the highresistance layer and induces current to flow in path 580 and provideenergy to the phase change material of memory element 548.

As represented heuristically in curve 1260 of FIG. 12, the pulse shapeof the retry bias arrangement is adapted to breakthrough the highresistance layer and create a conduction path. Because of the trailingedge, the pulse shape is also adapted to cause at least a portion of theactive region to transition into a crystalline phase, therebyestablishing the lower resistance state. As will be understood, thecurve 1260 is merely illustrative and the actual shape of the curve 1260depends upon the properties of the memory cell, the manner in which thesubsequent bias arrangement is applied to the memory cell, and themanner in which the phase change material heats up and cools down.

In some embodiments the amount of energy provided to the phase changematerial in step 1040 is sufficient to melt the active region of thephase change material and to cause a transition of at least an activeregion into a crystalline phase. In some alternative embodiments theenergy provided to the phase change material in step 1040 is sufficientmelt a portion of the phase change material greater than the activeregion, which may be useful for overcoming the set failure caused bycompositional changes of the phase change material within the activeregion.

In the illustrated embodiment of FIGS. 10-12, the retry bias arrangementof step 1040 comprises a single pulse as shown applied to the bit line560, although it will be understood that other retry bias arrangementscan alternatively be used. More generally, the retry bias arrangement ofstep 1040 may comprise a set of one or more pulses applied to the bitline 560 and/or word line 558 and/or source line 554 to induce currentflow in path 580. The number of pulses and the pulse shapes, includingthe voltage levels and pulse times, of the subsequent bias arrangementcan be determined empirically for each embodiment.

In the set operations 600, 1000 of FIGS. 6 and 10 the description refersto a single memory cell being programmed, although it will be understoodthat the set operations described herein are also applicable toprogramming a plurality of memory cells.

FIG. 13 illustrates an embodiment of set operation 1300 on a group ofcells of array 405. In the following discussion the various biasarrangements can be implemented as described above including usingpulses such as those described above, and thus a discussion of thevarious pulses and the bias arrangements of the set operation 1300 willnot be repeated here.

The set operation 1300 for a group of memory cells of array 405 beginsat step 1310. Step 1310 may include, or in some embodiments be precededby, a read operation.

Next at step 1320 a first bias arrangement is applied to the group ofmemory cells to establish the lower resistance state in the memorycells.

At step 1330 the resistances of the memory cells are read to determinewhether memory cells in the group of memory cells have respectiveresistances corresponding to the lower resistance state.

For memory cells in the group of memory cells not having a resistancecorresponding to the lower resistance state, those memory cells haveexperienced a set failure and a retry (or second) bias arrangement isapplied to those failed memory cells.

In the illustrated embodiment of FIG. 13, the retry bias arrangement ofthe set operation 1300 begins at step 1340 where a subsequent biasarrangement is applied to the memory cell.

Next, the retry bias arrangement of the set operation 1300 continuesback to step 1320 where the first bias arrangement is applied to thosefailed memory cells.

The set operation 1300 then continues to step 1330 to determine whetherthe failed memory cells have a resistance corresponding to the lowerresistance state. The set operation continues in the loop of iterativelyapplying the retry bias arrangement (steps 1340, 1320) to memory cellswhich again fail step 1330 and determining whether the failed memorycells from the preceding step 1330 have a resistance corresponding tothe lower resistance state (step 1330) until the resistance of thememory cells corresponds to the lower resistance state, or until amaximum number of retries are made. If in step 1330 it is determinedthat each of the memory cells in the group has a resistancecorresponding to the lower resistance state, the set operationterminates at step 1350.

In the set operation of FIG. 13, the retry bias arrangement comprisesthe subsequent bias arrangement of step 1340 combined with the firstbias arrangement of step 1320. Alternatively, similar to the discussionabove with respect to FIG. 10, the retry bias arrangement in someembodiments does not include the set bias arrangement of step 1320, andinstead includes the function of setting the group of failed memorycells.

Memory devices and methods for operating such devices described hereinaddress the set failure mode and result in improved endurance,reliability and improved data storage performance. Set operationsdescribed herein provide lower energy to the phase change material ofmemory cells to establish the lower resistance state and only applyhigher energy to the phase change material when the lower energy isinsufficient to set the memory cell. Thus, compared to melt-and-annealset methods, set operations described herein reduce the amount of highcurrent operations and thus improve the reliability of the memory cells.

Embodiments of the memory cells described herein include phase changebased memory materials, including chalcogenide based materials and othermaterials, for the memory elements. Chalcogens include any of the fourelements oxygen (O), sulfur (S), selenium (Se), and tellurium (Te),forming part of group VIA of the periodic table. Chalcogenides comprisecompounds of a chalcogen with a more electropositive element or radical.Chalcogenide alloys comprise combinations of chalcogenides with othermaterials such as transition metals. A chalcogenide alloy usuallycontains one or more elements from group IVA of the periodic table ofelements, such as germanium (Ge) and tin (Sn). Often, chalcogenidealloys include combinations including one or more of antimony (Sb),gallium (Ga), indium (In), and silver (Ag). Many phase change basedmemory materials have been described in technical literature, includingalloys of: Ga/Sb, In/Sb, In/Se, Sb/Te, Ge/Te, Ge/Sb/Te, In/Sb/Te,Ga/Se/Te, Sn/Sb/Te, In/Sb/Ge, Ag/In/Sb/Te, Ge/Sn/Sb/Te, Ge/Sb/Se/Te andTe/Ge/Sb/S. In the family of Ge/Sb/Te alloys, a wide range of alloycompositions may be workable. The compositions can be characterized asTe_(a)Ge_(b)Sb_(100−(a+b)). One researcher has described the most usefulalloys as having an average concentration of Te in the depositedmaterials well below 70%, typically below about 60% and ranged ingeneral from as low as about 23% up to about 58% Te and most preferablyabout 48% to 58% Te. Concentrations of Ge were above about 5% and rangedfrom a low of about 8% to about 30% average in the material, remaininggenerally below 50%. Most preferably, concentrations of Ge ranged fromabout 8% to about 40%. The remainder of the principal constituentelements in this composition was Sb. These percentages are atomicpercentages that total 100% of the atoms of the constituent elements.(Ovshinsky 5,687,112 patent, cols. 10-11.) Particular alloys evaluatedby another researcher include Ge2Sb2Te5, GeSb2Te4 and GeSb4Te7 (NoboruYamada, “Potential of Ge—Sb—Te Phase-Change Optical Disks forHigh-Data-Rate Recording”, SPIE v.3109, pp. 28-37 (1997).) Moregenerally, a transition metal such as chromium (Cr), iron (Fe), nickel(Ni), niobium (Nb), palladium (Pd), platinum (Pt) and mixtures or alloysthereof may be combined with Ge/Sb/Te to form a phase change alloy thathas programmable resistive properties. Specific examples of memorymaterials that may be useful are given in Ovshinsky '112 at columns11-13, which examples are hereby incorporated by reference.

Chalcogenides and other phase change materials are doped with impuritiesin some embodiments to modify conductivity, transition temperature,melting temperature, and other properties of memory elements using thedoped chalcogenides. Representative impurities used for dopingchalcogenides include nitrogen, silicon, oxygen, silicon dioxide,silicon nitride, copper, silver, gold, aluminum, aluminum oxide,tantalum, tantalum oxide, tantalum nitride, titanium and titanium oxide.See, e.g., U.S. Pat. No. 6,800,504, and U.S. Patent ApplicationPublication No. U.S. 2005/0029502.

Phase change alloys are capable of being switched between a firststructural state in which the material is in a generally amorphous solidphase, and a second structural state in which the material is in agenerally crystalline solid phase in its local order in the activechannel region of the cell. These alloys are at least biostable. Theterm amorphous is used to refer to a relatively less ordered structure,more disordered than a single crystal, which has the detectablecharacteristics such as higher electrical resistivity than thecrystalline phase. The term crystalline is used to refer to a relativelymore ordered structure, more ordered than in an amorphous structure,which has detectable characteristics such as lower electricalresistivity than the amorphous phase. Typically, phase change materialsmay be electrically switched between different detectable states oflocal order across the spectrum between completely amorphous andcompletely crystalline states. Other material characteristics affectedby the change between amorphous and crystalline phases include atomicorder, free electron density and activation energy. The material may beswitched either into different solid phases or into mixtures of two ormore solid phases, providing a gray scale between completely amorphousand completely crystalline states. The electrical properties in thematerial may vary accordingly.

Phase change alloys can be changed from one phase state to another byapplication of electrical pulses. It has been observed that a shorter,higher amplitude pulse tends to change the phase change material to agenerally amorphous state. A longer, lower amplitude pulse tends tochange the phase change material to a generally crystalline state. Theenergy in a shorter, higher amplitude pulse is high enough to allow forbonds of the crystalline structure to be broken and short enough toprevent the atoms from realigning into a crystalline state. Appropriateprofiles for pulses can be determined, without undue experimentation,specifically adapted to a particular phase change alloy. In followingsections of the disclosure, the phase change material is referred to asGST, and it will be understood that other types of phase changematerials can be used. A material useful for implementation of a PCRAMdescribed herein is Ge₂Sb₂Te₅.

An exemplary method for forming chalcogenide material uses chemicalvapor deposition CVD such as that disclosed in US Publication No2006/0172067 entitled “Chemical Vapor Deposition of ChalcogenideMaterials”, which is incorporated by reference herein.

A post-deposition annealing treatment in a vacuum or in an N2 ambient isoptionally performed to improve the crystallize state of chalcogenidematerial. The annealing temperature typically ranges from 100° C. to400° C. with an anneal time of less than 30 minutes.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is to be understood thatthese examples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations willreadily occur to those skilled in the art, which modifications andcombinations will be within the spirit of the invention and the scope ofthe following claims. What is claimed is:

1. A method for operating a memory cell being programmable to aplurality of resistance states including a high resistance state and alower resistance state, the method comprising: applying a first biasarrangement to the memory cell to establish the lower resistance state,the first bias arrangement comprising a first voltage pulse; and if thememory cell is not in the lower resistance state, then applying a secondbias arrangement to the memory cell to establish the lower resistancestate, the second bias arrangement comprising a second voltage pulsehaving a pulse height greater than that of the first voltage pulse. 2.The method of claim 1, wherein the second bias arrangement is sufficientto breakthrough a high resistance layer within an electrical conductionpath of the memory cell.
 3. The method of claim 1, further comprising:determining whether the memory cell is in the lower resistance stateafter applying the second bias arrangement to the memory cell; and ifthe memory cell is not in the lower resistance state after applying thesecond bias arrangement to the memory cell, iteratively applyingsubsequent bias arrangements to the memory cell and determining whetherthe memory cell is in the lower resistance state until the memory cellis in the lower resistance state or a predetermined number of retriesare made, wherein the subsequent bias arrangements respectively comprisea corresponding voltage pulse having a pulse height greater than that ofthe first voltage pulse to establish the lower resistance state.
 4. Themethod of claim 3, wherein the applying subsequent bias arrangements tothe memory cell comprises applying the second bias arrangement to thememory cell.
 5. The method of claim 1, wherein the determining whetherthe memory cell is in the lower resistance state comprises applying aread bias arrangement to the memory cell and detecting a current in thememory cell, the current in the memory cell corresponding to theresistance state of the memory cell.
 6. The method of claim 1, wherein:the memory cell further comprises an access device having a firstterminal coupled a word line and a second terminal coupled to a bit linevia the phase change material; the applying the first bias arrangementcomprises applying a voltage to the word line and applying the firstvoltage pulse to the bit line to induce a first current through thephase change material; and the applying the second bias arrangement tothe memory cell comprises applying a voltage to the word line andapplying the second voltage pulse to the bit line to induce a secondcurrent through the phase change material.
 7. The method of claim 6,wherein the second current through the phase change material is lessthan the first current.
 8. The method of claim 1, wherein applying thesecond bias arrangement comprises: applying the second voltage pulseacross the phase change material; and after applying the second voltagepulse, applying the first voltage pulse across the phase changematerial.
 9. The method of claim 1, wherein the second voltage pulse issufficient to melt an active region of the phase change material. 10.The method of claim 1, wherein the second voltage pulse has a pulseshape adapted to melt at least an active region of the phase changematerial and to cause a transition of at least a portion of the activeregion into a crystalline phase.
 11. A memory device comprising: amemory cell being programmable to a plurality of resistance statesincluding a high resistance state and a lower resistance state; and biascircuitry adapted to apply bias arrangements to the memory cell, thebias arrangements including: a first bias arrangement to establish thelower resistance state, the first bias arrangement comprising a firstvoltage pulse; and a second bias arrangement to establish the lowerresistance state if the memory cell is not in the lower resistance stateafter the first bias arrangement, the second bias arrangement comprisinga second voltage pulse having a pulse height greater than that of thefirst voltage pulse.
 12. The memory device of claim 11, wherein thesecond bias arrangement is sufficient to breakthrough a high resistancelayer within an electrical conduction path of the memory cell.
 13. Thememory device of claim 11, wherein the bias arrangements furtherinclude: a read bias arrangement to determine whether the memory cell isin the lower resistance state after the second bias arrangement; and asubsequent bias arrangement to establish the lower resistance state ifthe memory cell is not in the lower resistance state after the secondbias arrangement, the subsequent bias arrangement comprising a voltagepulse having a pulse height greater than that of the first voltagepulse.
 14. The memory device of claim 13, wherein the subsequent biasarrangement comprises the second bias arrangement.
 15. The memory deviceof claim 11, wherein: the memory cell further comprises an access devicehaving a first terminal coupled to a word line and a second terminalcoupled to a bit line via the phase change material; the first biasarrangement comprises a voltage applied to the word line and the firstvoltage pulse applied to the bit line to induce a first current throughthe phase change material; and the second bias arrangement comprises avoltage applied to the word line and the second voltage pulse applied tothe bit line to induce a second current through the phase changematerial.
 16. The memory device of claim 15, wherein the second currentthrough the phase change material is less than the first current. 17.The memory device of claim 11, wherein the second bias arrangementcomprises a plurality of voltage pulses.
 18. The memory device of claim12, wherein the second bias arrangement comprises: the second voltagepulse across the phase change material; and after the second voltagepulse, the first voltage pulse across the phase change material.
 19. Thememory device of claim 11, wherein the second voltage pulse issufficient to melt an active region of the phase change material. 20.The memory device of claim 11, wherein the second voltage pulse has apulse shape adapted to melt at least an active region of the phasechange material and to cause a transition of at least a portion of theactive region into a crystalline phase.